Device for gas-sensoring electrodes

ABSTRACT

A sensing element, in particular for an electrochemical sensor for determining gas concentrations, having at least one three-dimensional electrode arrangement, applied on a support plate and forming trenches of a depth for measuring changes in capacitance and/or conductivity in a gas-sensitive layer arranged to a height in the trenches, the height of the gas-sensitive layer being less than the depth of the trenches.

FIELD OF THE INVENTION

The present invention is based on a sensing element, in particular foran electrochemical measurement sensor for determining gasconcentrations.

BACKGROUND INFORMATION

The use of planar electrode arrangements for chemical sensors is known.The gas concentrations are ascertained by determining changes incapacitance and/or conductivity in a gas-sensitive material. Also knownis the three-dimensional configuration of electrode arrangements, whichfurther increases the sensitivity of chemical sensors (Lin et al.,Sensors and Actuators 5 (1991), 223 to 226). According to Lin et al.,the manufacture of three-dimensional electrode arrangements isaccomplished by first sputtering a metallic film onto a siliconsubstrate and then patterning a photoresist applied upon patterning ofthe photoresist, the resulting trenches are filled by electroplating,yielding a three-dimensional electrode structure as an inverse resiststructure. After removal of the photoresist, the trenches, i.e, theelectrode interstices, are filled with a gas-sensitive substance.

SUMMARY

The sensing element has, the advantage that the three-dimensionalstructure of the electrode arrangement can be used as a retainingstructure for catalytically active layers and/or protective layers, andlocally as a wall catalyst. Because the gas-sensitive materialintroduced into the trenches does not fill them up completely, it is onthe one hand possible to cover over the gas-sensitive material withcatalyst layers and/or protective layers, and/or on the other hand touse regions of the three-dimensional electrode arrangement which are notcovered by gas-sensitive material or other layers as a wall catalyst. Inthe embodiments according to the present invention in which thegas-sensitive material is covered over with protective layers and/orcatalytically active layers, the three-dimensional structure of theelectrode arrangement acts as a retaining structure for those layers,and guarantees a stable configuration for the sensing element. In theembodiments according to the present invention in which the inner walls,i.e. the walls forming the trenches, of the three-dimensional electrodearrangement are not completely covered by the aforesaid layers whichcover the gas-sensitive material, the inner walls can be used as a wallcatalyst. The use of catalytic layers covering the gas-sensitivematerial and/or the use of the inner walls of the three-dimensionalelectrode arrangement as a wall catalyst is advantageous, and after“particular” in particular if the gas-sensitive material does notexhibit complete selectivity for the gas to be measured. In such a caseit is particularly desirable to subject the gas mixture beinginvestigated to catalysis, the gas to be detected being catalyticallyconverted in such a way that it is detected by the gas-sensitive layerand determined as selectively as possible. According to the presentinvention, improved selectivity of the gas measurement in thegas-sensitive material can be achieved by using a catalytically activelayer and/or by wall catalysis. The use of additional catalyticallyactive layers can be dispensed with, since the conversion into thespecific gas to be detected is accomplished by inner-wall catalysis.

The present invention also provides that the height h of thegas-sensitive layer introduced into the trenches, or the depth T of thetrenches, can vary, although in every region of the trench the height his to be substantially less than depth T of the trenches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1 h show in schematic sequence, a process steps formanufacturing a sensing element which includes three-dimensionalminiaturized electrode arrangements according to one embodiment of thepresent invention, the sensing element being depicted in longitudinalsections .

FIG. 1a shows a step of applying an electroplating starter layer.

FIG. 1b shows a step of applying a photoresist layer.

FIG. 1c shows a step of transferring a metallic three-dimensionalelectrode arrangement into the photoresist layer.

FIG. 1d shows a step of depositing metal into resist trenches.

FIG. 1e shows a step of dissolving the photoresist layer.

FIG. 1f shows a step of producing a heating electrode.

FIG. 1g shows a step of removing electroplating starter layers.

FIG. 1h shows a step of placing a paste into interstices betweenelectrodes.

FIG. 2 schematically shows, in a two-dimensional depiction, ameander-shaped, three-dimensional, four-pole electrode arrangement.

FIG. 3 schematically shows, in a two-dimensional depiction, arectangular, three-dimensional, two-pole electrode arrangement.

FIG. 4 schematically shows, in a two-dimensional depiction, aspiral-shaped, three-dimensional, two-pole electrode arrangement.

FIG. 5 schematically shows the use of inner walls of thethree-dimensional electrode arrangement, depicted in longitudinalsection, for wall catalysis.

FIG. 6 schematically shows the use of the three-dimensional electrodearrangement, depicted in longitudinal section, as a retaining structurefor catalyst layers and protective layers.

FIG. 7 shows a sensor array in a 2×2 arrangement.

FIG. 8 shows a sensor array in a cloverleaf arrangement.

FIG. 9 shows a sensor array for temperature-dependent measurements.

DETAILED DESCRIPTION

FIGS. 1a-1 h shows, in schematic sequence, the process steps a) throughh) for manufacturing three-dimensional miniaturized electrodearrangements. In step a), (FIG. 1a), after a cleaning step, anelectroplating starter layer 4 is applied by sputtering onto a flatsupport plate 2. Especially if the sensor to be manufactured is to beused in highly corrosive media, for example in exhaust gas diagnosis,support plate 2 can be made Al₂O₃, onto which platinum is applied as theelectroplating starter layer. If the requirements in terms of corrosionresistance are less stringent, other substrates such as silicon or glasscan be used, and metals such as gold, silver, copper, chromium, andothers can be used to manufacture the electroplating starter layer. Ifthe sensors are to be integrated with an electronic analysis system, theuse of silicon substrates is particularly advantageous.

In step b), (FIG. 1b), a photoresist layer 6, for example a photoresist,polyimide, or solid resist, is then applied onto the entire surface ofsupport plate 2, by spin-coating (for liquid resists) or by rolling (forsolid resists). The film thickness of photoresist layer 6 is adjusted byway of the rotation speed for liquid resists, and by way of the numberof rolled-on resist layers, for solid resists. According to an exampleembodiment film thicknesses between 10 μm and 100 μm may be used.

In step c), (FIG. 1c) the metallic three-dimensional electrodearrangement that is to be manufactured is transferred inversely intophotoresist layer 6 by way of a photolithographic mask. In a deep UVlithographic method, the resist is illuminated directly through a mask.Another possibility is to deposit onto the photoresist an oxide, anitride, or a metal which is photolithographically patterned as a maskfor a dry etching process of photoresist layer 6. Smaller pattern widthscan be manufactured with the dry etching process than with the deep UVlithographic method. Both alternatives result in the formation of resisttrenches 8 in photoresist layer 6.

In step d), (FIG. 1d) metal is deposited into resist trenches 8; resisttrenches 8 can be filled up to their upper edge. By varying thethickness of metal layer 10, it is possible to adjust the sensorsensitivity in controlled fashion. Selection of the material to bedeposited depends on the corrosion resistance required for the sensor:platinum, gold, and silver are thus possible for stringent requirements,and metals such as copper, nickel, or the like for lesser requirements.

In step e), (FIG. 1e) photoresist layer 6 is dissolved out of theapplied metal structure 10, resulting in free-standing three-dimensionalelectrode structures. Alkaline solutions such as a potassium hydroxidesolution, or organic solvents such as acetone, can be used depending onthe photoresist that is utilized.

In step f), (FIG. 1f) according to the example embodiment of the presentinvention a heating electrode can be produced on rear side 12 of supportplate 2 so that the sensor can be kept at a constant temperature. Thegeometry of heating electrode 10′ is defined by a mask pattern, andpatterning is performed as described in steps a) through e).

In step g), (FIG. g) electroplating starter layers 4 and 4′ are removedin order to interrupt the conductive connections between electrodes 10of the sensor and also of heater 10′. The electroplating starter layersare removed by etching them away, for example by wet-chemical etching,anionic etching, or a dry etching process.

In step h), (FIG. h) a paste is placed into the interstices betweenelectrodes 10 using the screen-printing method; this is then sintered atseveral hundred degrees, which forms layer 14 containing thegas-sensitive material. The paste is layered to a specific height hwhich is less than the depth T of the trenches and or thethree-dimensional electrodes 10. Further layers, for example protectivelayers or catalytically active layers, can be applied over gas-sensitivelayer 14 between electrodes 10, as depicted in FIG. 6. Especially whenplatinum is used as the electrode material, inner walls 16 of electrodes10, which are not covered by gas-sensitive layer 14, can be used forcatalysis K, as depicted in FIG. 5.

FIGS. 2-4 and 7-9 schematically shown, in plan view, example electrodearrangements which effectively utilize the entire surface of the sensor.Although the depiction is two-dimensional, the electrode arrangementsdepicted have a three-dimensional form. Functionally identicalstructures are labeled with identical reference numbers.

FIG. 2 depicts a three-dimensional electrode arrangement with four-polegeometry according to an example embodiment of the present invention.Four individual electrodes 18, 20, 22, and 24, which correspondinglyallow four-pole measurements, are depicted. Four-pole measurement offersthe advantage over two-pole measurement that any contact resistanceswhich occur are sensed instrumentally and can thus be eliminated. It isalso evident from FIG. 2 that electrodes 18, 20, 22, and 24 are coiledfor effective surface area utilization; the condition that the sameelectrodes must always face one another must be observed. Otherwiseleakage currents would occur, decreasing the sensor sensitivity. FIG. 2depicts an electrode arrangement with a meander structure, in which thefour electrodes 18, 20, 22, and 24 are uninterrupted. In addition tothis rectilinearly arranged coiling, any other electrode geometries,with curved or zig-zagging layouts, can also be provided in accordancewith the present invention.

FIG. 3 depicts a three-dimensional electrode arrangement of electrodes18′ and 20′ in a two-pole geometry according to an example embodiment ofthe present invention. The electrodes are arranged in meander fashion,the electrodes running in a rectilinear internally coiled shape.

FIG. 4 depicts a three-dimensional arrangement of electrodes 18′ and 20′in two-pole geometry with a spiral electrode layout according to anexample embodiment of the present invention. As in the previous figures,the purpose of the electrode structure is to achieve good surface areautilization on the support plate. The electrode layout can of course beadapted to the lateral heat distribution on the substrate, so that thesensor region can be laid exactly on an isothermal surface.

FIG. 5 illustrates that fill height h of gas-sensitive layer 14 is lessthan depth T of trenches 26 enclosed by electrodes 10. Inner walls 16 ofelectrodes 10, which are not covered by gas-sensitive material 14, arepreferably catalytically active, especially when platinum is used as theelectrode material. The gas to be detected is catalytically converted onthe inner walls so that it can be detected by gas-sensitive layer 14located therebeneath.

FIG. 6 illustrates a further embodiment of the present invention, inwhich two further layers have been applied over the gas-sensitive layer.Gas-sensitive layer 14, filled up to a height h, is covered by a layer28 which catalytically converts the gas that is to be detected, so thatit can be sensed in layer 14. Arranged above catalytically active layer28 is a protective layer or cover layer 30, which protects theunderlying layers 28 and 14 from external influences such as moistureand dirt. The three-dimensional electrode structure thus serves here asa retaining structure for catalytically active layer 28 and cover layer30.

FIG. 7 shows the combination of three-dimensional miniaturized electrodearrangements into a 2×2 region. The individual electrodes are labeled30.1 through 30.8.

FIG. 8 shows the grouping of one three-dimensional electrode structureinto a four-fold structure with a central tap which is embodied in acoiled arrangement. The four individual sensors can thereby be spatiallyresolved, i.e. operated so that, for example, influences of a gas flowcan be compensated for. Other electrode arrangements in a cloverleafstructure are of course also possible in any desired geometry, forexample as round and elliptical coils.

FIG. 9 illustrates the arrangement of individual sensors along a definedtemperature gradient T. This embodiment makes possibletemperature-dependent measurements by individual interrogation of thesensors. The temperature gradient T is defined by the heater on the rearside of support plate 2.

Configuration of the sensor arrays depicted in FIGS. 7 through 9 is madepossible in particular by the miniaturization made possible bythree-dimensional patterning. Arrangement in arrays can make possiblespatially resolved measurements, and the detection of different gases bythe use of multiple gas-sensitive substances.

What is claimed is:
 1. A sensing element, comprising: a support plate;at least one three-dimensional arrangement of electrodes applied on thesupport plate, the arrangement of electrodes forming trenches of a depthof between 10 μm and 100 μm; a gas-sensitive layer arranged to apredetermined height only in the trenches, the arrangement of electrodesmeasuring changes in at least one of potential, capacitance andconductivity in the gas sensitive layer, the predetermined height of thegas-sensitive layer being less than the depth of the trenches; and acatalytically active layer covering the gas sensitive layer.
 2. Thesensing element according to claim 1, further comprising: a protectivelayer that is arranged in the trenches above one of the gas-sensitivelayer and the catalytically active layer.
 3. The sensing elementaccording to claim 1, wherein the electrodes of the at least onethree-dimensional arrangement of electrodes are arranged in the form ofan interdigitated structure.
 4. The sensing element according to claim1, wherein the at least one three-dimensional arrangement of electrodesis formed from four individual electrodes, which allow a four-polemeasurement.
 5. The sensing element according to claim 1, wherein thesupport plate includes one of ceramic material, glass, aluminum oxideand a silicon/silicon dioxide mixture.
 6. The sensing element accordingto claim 1, wherein the electrodes of the at least one arrangement ofelectrodes includes one of platinum, gold, silver, copper and nickel. 7.A sensing element, comprising: a support plate; at least twothree-dimensional arrangements of electrodes applied on the supportplate, the arrangements of electrodes forming trenches of a depth ofbetween 10 μm and 100 μm; a gas-sensitive layer arranged to apredetermined height only in the trenches, the arrangements ofelectrodes measuring changes in at least one of potential, capacitanceand conductivity in the gas sensitive layer, the predetermined height ofthe gas-sensitive layer being less than the depth of the trenches; and acatalytically active layer covering the gas sensitive layer.
 8. Anelectrochemical sensor for determining a gas concentration, comprising:at least one of a sensing element and sensor array, the at least onesensing element and sensor array, including a support plate, at leastone three-dimensional arrangement of electrodes applied on the supportplate, the arrangement of electrodes forming trenches of a depth ofbetween 10 μm and 100 μm, a gas-sensitive layer arranged to apredetermined height only in the trenches, the arrangement of electrodesmeasuring changes in at least one of potential, capacitance andconductivity in the gas sensitive layer, the predetermined height of thegas-sensitive layer being less than the depth of the trenches, and acatalytically active layer covering the gas sensitive layer.
 9. A methodfor manufacturing a sensing element having a three-dimensional electrodearrangement, comprising the steps of: depositing an electroplatingstarter layer onto a support plate; applying a photoresist at athickness of between 10 μm to 100 μm onto the electroplating starterlayer; patterning the photoresist to form resist trenches; filling theresist trenches by electroplating up to a defined height; removing thephotoresist to form electrode trenches; etching away the electroplatingstarter layer from within the electrode trenches; placing a gassensitive material into the electrode trenches up to a predeterminedheight that is less than the depth of the electrode trenches; andapplying a catalytically active material onto the gas sensitive materialin the electrode trenches.